Integrate import thread pool with others

[joshmoore]

see: https://github.com/ome/omero-blitz/pull/65/files#r321705576

            keepAlive = new ClientKeepAlive();
            keepAlive.setClient(this); // This is used elsewhere.
            executor = new ScheduledThreadPoolExecutor(1);
            executor.scheduleWithFixedDelay(keepAlive, keepAliveRate, keepAliveRate, TimeUnit.SECONDS);